Variable attenuator employing internal switching



5. F. ADAM May 9, 1967 y' VARIABLE ATTENUATOR EMPLOYING INTERNAL SWITCHING Filed Oct. 8, 1965' myENToR STEPHEN F. ADAM BY' Q .CQ m

ATTORNEY United States Patent O Y 3,319,194 VARIABLE ATTENUATOR EMPLOYING INTERNAL SWITCHING Stephen F. Adam, Los Altos, Calif., assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California Filed Oct. 8, 1965, Ser. No. 493,985 3 Claims. (Cl. 333--S1) This invention relates to a high frequency signal attenuator which provides discrete steps of attenuation using separate attenuator elements which are all disposed in a transmission line configuration adjacent a common and continuous ground plane. An attenuator of this type obviates the need for complex mechanisms for switching both the signal and ground plane conductors of the transmission line structure and thus eliminates the introduction of unknown contact irnpedances in the ground plane conductor at the junctions of attenuator sections.

Accordingly, it is an object of the present invention to provide an improved step attenuator for high frequency signals.

In accordance with the illustrated embodiment of the present invention, a strip line structure is formed in a continuous ground plane conductor using a number of switchable sections, each including a resistive card attenuator and a straight-through conductor. Selection of either of the two signal paths is accomplished by defiecting the signal conductor from contact with one signal path to contact with the other signal path using magnetic or mechanical actuators. The effects of leakage paths `around the selected attenuating signal path may be overcome by establishing shorting paths simultaneously with the deflection of the signal conductor.

'I'hese and other objects of the present invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is a front sectional view of the attenuator according to the present invention;

FIGURE 2 is an end sectional view of the attenuator of FIGURE 1; and

FIGURE 3 is a top sectional view of the attenuator of FIGURE 1.

In FIGURES 1, 2 and 3 of the drawing, there is shown a body 9 which forms the ground plane conductor of a strip line. Coaxial connectors 11, 13 at the ends of the body 9 each include a center .conductor 15 which is matched coupled to the strip line conductor 17 and an outer conductor 19 which is connected to the body 9. The strip line conductor 17 is supported on a dielectric slab 21 which is mounted in longitudinal grooves 23 in the side walls of the ground plane conductor or body 9.

At selected intervals along the length of the strip line conductor, a parallel pair of signal conductive elements 25 and 27 are disposed within the ground plane conductor 9 above and below the plane of the strip line conductor 17. The lower conductive element 25 forms a straight-through transmission path and includes a conductive strip line 29 supported by a dielectric slab 31 which is mounted in longitudinal slots 33. The width of the strip line 29 is decreased to maintain the characteristic impedance of the transmission line which is formed with closer spacing to the ground plane conductor 9. The upper conductive element forms an attenuating transmission path and includes a resistive film 35 which is disposed on a dielectric slab 37 mounted in longitudinal slots 39 and which is connected to the ground plane conductor 9 along its longitudinal edges 41. Contacts 43, 45 with the resistive film at restricted areas 3,3l9,l94 Patented May 9, 1967 ice at the ends of the slab 37 provide connection points for the signal transmission path. A resistive card attenuator of this type is described in connection with a coaxial transmission line in pending .U.S. patent application Ser. No. 393,127 tiled Aug. 31, 1964 by W. R. Hewlett and W. B. Wholey and issued on Jan. 4, 1966 as U.S. Patent 3,227,975. The strip line conductor 17 includes a iiexible portion 47 at each side of the parallel pair of signal transmission paths which serves as a switching element and which is tapered to maintainthe characteristic impedance of the transmission line. The switching element 47 is actuated either magnetically by suitable electromagnetics 49 and programming `power source 50 or mechanically by an actuator 51 and programming cam assembly 53. The actuator 51 may be any dielectric material which passes through an aperture in the ground plane conductor 9 that has dimensions which cause the aperture to operate as a waveguide beyond cutoi at the frequencies of signal applied to the attenuator so that signal leakage is negligible. A selected step of attenuation is provided by switching the strip line conductor 47 at both ends of the parallel pair of signal transmission paths to the resistive card path. When a plurality of such paths are provided, each with an attenuator card of selected value such as 5 db, 10 db, 20 db and 40 db, a number of attenuation steps in 5 db increments from 5 db to 75 db may be provided by selectively switching in either an attenuation transmission path or a straightthrough transmission path. This selection is provided in a conventional manner either by the programmed power source 50 (used with the magnetic actuators) or the cam assembly 53 (used with the mechanical actuator 51) in response to the position of the attenuation selector dial 55.

For higher attenuation sections, (e.g. 40 db or higher) spurious transmission paths around the attenuator card are formed along the closely-spaced straight-through transmission path of the parallel pair. This disturbs the attenuation vs. frequency response of the attenuator and may readily be suppressed by shorting slugs 57 which are brought into contact with the straight-through transmission path simultaneously with the magnetic or mechanical actuation of the switching elements of that attenuation section.

I claim:

1. Signal apparatus comprising:

a transmission line including a ground plane conductor;

a first signal transmission path of said transmission line within said ground plane conductor including a resistance layer on a dielectric;

means connecting the ground plane conductor and said resistive lm along the longitudinal edges thereof;

contacts at each end of said resistive film within a restricted area intermediate the longitudinal edges of the resistive film;

a second signal transmission path within the ground plane conductor in spaced plane-parallel relation to the resistance layer on a dielectric in the iirst signal transmission path;

another signal conductor at each end of the first and second signal transmission paths disposed intermediate the spacing thereof and within the ground plane conductor;

a switching element at each of the signal conductor adjacent the first and second signal transmission paths forming a portion of the length of said signal conductor; and

actuator means for simultaneously deecting said switching elements to the same one of said first and second signal transmission paths.

2. Signal apparatus comprising: 3. Signal apparatus as in claim 2 wherein:

a signal attenuating path and a signal conduction path each of said switching elements has a width which disposed in spaced relationship within a ground plane decreases with length from the signal conductor. conductor; t

a signal conductor at each end of the signal attenuat- 5 References Cited by the Examiner ing and signal conduction paths disposed inter- UNITED STATES PATENTS mediate the spacing thereof and within the ground 2 597 090 5/1952 Freeman 333 81 plane conductor;

a switching element attached at the end of each of the gnclelman signal conductors adjacent the signal attenuating and l0 3 087125 4/1963 shle 333:97

signal conduction paths forming a portion of the length of each of said signal conductors, each of ELI LIEBERMAN Primary Examiner the switching elements being capable of flexing over the length thereof; and HERMAN KARL SAALBACH, Examiner. actuator means for simultaneously deecting said 15 REHUNT, Assistant Examnen switching elements to the same one of said signal attenuating and vsignal conduction paths. 

1. SIGNAL APPARATUS COMPRISING: A TRANSMISSION LINE INCLUDING A GROUND PLANE CONDUCTOR; A FIRST SIGNAL TRANSMISSION PATH OF SAID TRANSMISSION LINE WITHIN SAID GROUND PLANE CONDUCTOR INCLUDING A RESISTANCE LAYER ON A DIELECTRIC; MEANS CONNECTING THE GROUND PLANE CONDUCTOR AND SAID RESISTIVE FILM ALONG THE LONGITUDINAL EDGES THEREOF; CONTACTS AT EACH END OF SAID RESISTIVE FILM WITHIN A RESTRICTED AREA INTERMEDIATE THE LONGITUDINAL EDGES OF THE RESISTIVE FILM; A SECOND SIGNAL TRANSMISSION PATH WITHIN THE GROUND PLANE CONDUCTOR IN SPACED PLANE-PARALLEL RELATION TO THE RESISTANCE LAYER ON A DIELECTRIC IN THE FIRST SIGNAL TRANSMISSION PATH; ANOTHER SIGNAL CONDUCTOR AT EACH END OF THE FIRST AND SECOND SIGNAL TRANSMISSION PATHS DISPOSED INTERMEDIATE THE SPACING THEREOF AND WITHIN THE GROUND PLANE CONDUCTOR; A SWITCHING ELEMENT AT EACH OF THE SIGNAL CONDUCTOR ADJACENT THE FIRST AND SECOND SIGNAL TRANSMISSION PATHS FORMING A PORTION OF THE LENGTH OF SAID SIGNAL CONDUCTOR; AND ACTUATOR MEANS FOR SIMULTANEOUSLY DEFLECTING SAID SWITCHING ELEMENTS TO THE SAME ONE OF SAID FIRST AND SECOND SIGNAL TRANSMISSION PATHS. 